Industrial types of direct current (DC) thermal spray plasma torches are built with a water-cooled tungsten cathode and a copper anode. Main plasma gas is argon. The use of argon is dictated by its inertness at high temperatures to the thermionic tungsten cathode. Thermionic cathodes emit electrons from their surface since their temperature is high enough for easy emission of electrons. Tungsten is the preferred cathode material since it is a refractory metal with high melting point temperature. It is however, highly reactive to oxygen at high temperatures. During the operation of the torch, cathode tip is melted and tungsten evaporates. The cathode erosion rate is directly dependent on its temperature. Cathode lifetime and consistency of its performance is an important issue in this technology.
One disadvantage of argon is its low thermal conductivity and its low enthalpy which results in reduced thermal efficiency of the DC plasma torches. The low thermal efficiency limits powder feed rate, deposition efficiency and coating quality. To enhance thermal conductivity and thermal efficiency, small amounts of hydrogen or helium are normally mixed with argon.
It is known that to reduce the erosion of the graphite cathodes, they must be cooled either by encasing them in a water-cooled metal jacket (see for example U.S. Pat. Nos. 4,490,825 and 4,304,980) or by external water spraying directly onto the electrode (U.S. Pat. No. 5,795,539). Direct internal water cooling of graphite electrodes is not practical since the cathode is normally made of polycrystalline graphite which has open porosity and, compared to metals, lower thermal conductivity. This leads to the infiltration of the cooling water through the electrode as well as a less effective heat removal. The latter imposes limits on power generated by the plasma torch.
It would be very advantageous to provide a DC plasma torch which has a long lasting thermionic cathode having a high thermal efficiency.